Theory
The pH is the measure of the acidic (or basic) power of a solution. It is a scale for measuring hydrogen ion concertration in a solution. The pH scale varies from 0 to 14. At 25°C (298 K), a neutral solution has pH equal to 7. A value less than 7 on the pH scale represents an acidic solution whereas basic solution has pH value more than 7.
pH is defined as the negative logarithm(base 10) of the hydrogen ion concentration in moles per litre.
pH = -log10[H+]

Step 1: Six clean test tubes is placed in a test tube stand.
Step 2: Dilute HCl, Dilute NaOH Solution, Dilute CH3COOH solution, Lemon juice, Water and Dilute NaHCO3 solution are taken separately in six test tubes and labelled.
Step 3: One or two drops of each test solution on different strips of pH papers are put using a glass rod. Glass rod used for one sample must be washed with water before used for the other sample.
Step 4: Note the pH by comparing the colour appeared on the pH paper with those on colour chart for pH paper.
5. For determining the pH of lemon juice, squeeze the fruit and place 1 or 2 drop of the juice on the pH paper.

Observations

Sl. No.

Sample Solution

Colour appeared on pH paper

pH of the solution (Approximate)

Inference

1.

Dilute HCl

Red

1

Strong acid

2.

Dilute NaOH Solution

Purple

14

Strong alkali

3.

Dilute CH3COOH solution

Yellow

3

Weak acid

4.

Lemon juice

Orange

2

Acid

5.

Water

Green

7

Neutral

6.

Dilute NaHCO3 solution

Blue

9

Weak alkali

Results and Discussions
As pH depends upon H+ concentration and in an aqueous solution H+ and OH- ion concentrations are correlated, therefore, every acidic and basic solution shows different colour at different pH.

Precautions
1. The test sample solutions should be freshly prepared and the fruit lemon juice samples should also be fresh.
2. Glass rod used for one sample should be used for the other sample only after washing it with water.
3. Acids must be handled carefully.